Active photosensing pixel with two-terminal photosensing transistor

ABSTRACT

An active photosensing pixel is disclosed, in which a two-terminal photosensing transistor has a first terminal coupled to a first node, a second terminal coupled to a first selection line and a control terminal connected to the second terminal. A driving transistor has a first terminal coupled to a first reference voltage, a second terminal coupled to an output line and a control terminal connected to the first node. A reset transistor has a first terminal connected to the first node, a second terminal coupled to a second reference voltage and a control terminal coupled to a second selection line.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of Taiwan Patent Application No. 099119404, filed on Jun. 15, 2010, the entirety of which is incorporated by reference herein.

BACKGROUND

1. Field of the Disclosure

The present disclosure relates to a display, and in particular relates to a photosensing pixel display, wherein the photosensing pixel executes exposure and readout procedures at the same time when selection lines are selected.

2. Description of the Related Art

Lately, E-books have been developed and commercialized. One feasible display configuration for E-books, adopts a thin film transistor liquid crystal display (TFT-LCD) thereto. In other words, the E-book displays images by using electronic components, such as a TFT or photosensing device, disposed on a backplane thereof. E-books must have the ability to sense a photo so that marking on a screen by touching the screen may be accomplished. As an example, however, for an E-book with a photosensing function, since the photosensing device is installed under the backplane, light transmittance therethrough is decreased. Thus, E-Books need amount of time for exposure of marking on a screen thereof.

An active photosensing pixel which can quickly mark on screen of E-books is desired.

BRIEF SUMMARY

A detailed description is given in the following embodiments with reference to the accompanying drawings.

An embodiment discloses an active photosensing pixel, comprising a two-terminal photosensing transistor, a driving transistor and a reset transistor. The two-terminal photosensing transistor has a first terminal coupled to a first node, a second terminal coupled to a first selection line and a control terminal connected to the second terminal. The driving transistor has a first terminal coupled to a first reference voltage, a second terminal coupled to an output line and the control terminal connected to the first node. The reset transistor has a first terminal connected to the first node, the second terminal coupled to a second reference voltage and the control terminal connected to a second selection line.

An embodiment discloses another active photosensing array, comprising a plurality of selection lines, output lines and active photosensing pixels, wherein the selection lines comprise a first selection line and a second selection line. Each active photosensing pixel comprises a two-terminal photosensing transistor, a driving transistor and a reset transistor. The two-terminal photosensing transistor has a first terminal coupled to a first node, a second terminal coupled to a first selection line and a control terminal connected to the second terminal. The driving transistor has a first terminal coupled to a first reference voltage, a second terminal coupled to the corresponding output line and the control terminal connected to the first node. The reset transistor has a first terminal connected to the first node, the second terminal coupled to a second reference voltage and the control terminal connected to a second selection line.

An embodiment further discloses a photosensing method for an active photosensing pixel, wherein the active photosensing pixel comprises a two-terminal photosensing transistor having a first terminal coupled to a first node, a second terminal coupled to a first selection line and a control terminal connected to the second terminal, a driving transistor having a first terminal coupled to a first reference voltage, a second terminal coupled to an output line and a control terminal connected to the first node and a reset transistor having a first terminal connected to the first node, a second terminal coupled to a second reference voltage and a control terminal coupled to the second selection line. The photosensing method includes the following steps. First, a first voltage and a second voltage are provided to the first and the second selection lines, respectively, during an exposure and readout cycle, such that the two-terminal photosensing transistor is served as a diode, wherein the first voltage is higher than that of the second voltage. Next, a diode current is generated to charge the first node when the two-terminal transistor receives an incident light, such that the driving transistor is turned on according to a voltage level of the first node to produce an output current to the selection lines.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

FIG. 1A is a schematic view showing an embodiment of the first operation mode for the two-terminal photosensing transistor;

FIG. 1B is an embodiment of the relationship of the photosensing current I_(photo) and the voltage V_(N1) at the first terminal of the two-terminal photosensing transistor in the first operation mode;

FIG. 2A is a schematic view showing an embodiment of the second operation mode for the two-terminal photosensing transistor;

FIG. 2B is an embodiment of the relationship of the diode current I_(diode) and the voltage V_(N2) at the second terminal of the two-terminal photosensing transistor in the second operation mode;

FIG. 3 is a schematic view showing an embodiment of the active photosensing pixel;

FIG. 4 is a sequence diagram showing an embodiment of the selection lines and the voltage waveform of the first node;

FIG. 5 is a schematic view showing another embodiment of the active photosensing pixel;

FIG. 6 is a schematic view showing an embodiment of the active photosensing array and a sequence diagram of the corresponding selection lines;

FIG. 7 is a schematic view showing an embodiment of the display with an active photosensing array and a sequence diagram of the corresponding selection lines.

DETAILED DESCRIPTION

The embodiment will be explained as follows. The following description is made for the purpose of illustrating the embodiment and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.

FIG. 1A is a schematic view showing an embodiment of the two-terminal sensing transistor in a first operation mode. In this embodiment, the two-terminal photosensing transistor (written as Q₁ hereinafter) is an N-type a-Si:H TFT, but is not limited thereto. The two-terminal photosensing transistor Q₁ includes the first terminal N₁, the second terminal N₂ and a control terminal. Note that the control terminal of the two-terminal photosensing transistor Q₁ is connected to the second terminal N₂, wherein, it forms two terminals of the two-terminal photosensing transistor. That is, the first terminal N₁ and the control terminal are connected to the second terminal N₂. In the first operation mode, the high voltage V_(H) is applied to the first terminal of the two-terminal photosensing transistor Q₁, and the low voltage V_(L) is applied to the second terminal N₂. The two-terminal photosensing transistor Q₁ produces the photosensing current I_(photo) from the first terminal N₁ to the second terminal N₂ when the two-terminal photosensing transistor Q₁ receives the incident light hv in the first operation mode. Generally speaking, the intensity of the photosensing current I_(photo) is determined by the surface dimensions of the semiconductor layer and material of the two-terminal photosensing transistor Q₁. Moreover, the intensity of the photosensing current I_(photo) can also be determined by the intensity of the incident light hv. In other words, the stronger the incident light hv is, the greater the photosensing current I_(photo) will be. Therefore, the two-terminal photosensing transistor Q₁ is served as a photosensitive resistor in the first operation mode. For other embodiments, the two-terminal photosensing transistor Q₁ can be a P-type a-Si:H TFT, but is not limited thereto. In other embodiments, the two-terminal photosensing transistor Q₁ may be bipolar junction transistor (BJT) or other switching devices.

FIG. 1B shows, during the first operation mode, the relationship of the photosensing current I_(photo) and the voltage V_(N1) at the first terminal of the two-terminal photosensing transistor Q₁. As shown in FIG. 1B, the magnitude of the photosensing current I′_(photo) is zero (also named cut-off region) when there is no incident light hv (the line connected by diamond points as shown). On the contrary, when the two-terminal photosensing transistor Q₁ receives incident light hv (indicated as the line that connected by the square points), the photosensing current I_(photo), which produced by the two-terminal photosensing transistor Q₁, will linearly increase, before slowing down by degrees (known as the triode region), and finally, becoming saturated (known as the saturation region). Hence, the above description of the two-terminal photosensing transistor Q₁ is similar to the general field-effect transistor (FET). In one embodiment, the photosensing current I_(photo) is around 7.5E-09 A if the two-terminal photosensing transistor Q₁ receives the incident light and the level of the first terminal N₁ is 16V, and the photosensing current I′_(photo) is around 0 A if the two-terminal photosensing transistor Q₁ receives no incident light and the level of the first terminal N₁ is 16V. It can be determined whether the two-terminal photosensing transistor Q₁ has received the incident light by detecting the photosensing current I_(photo) during the first operation mode.

FIG. 2A is a schematic view showing an embodiment of the two-terminal photosensing transistor Q₁ under the second operation mode. Similar with FIG. 1A, the control terminal of the two-terminal photosensing transistor Q₁ is also connected to the second terminal N₂. Compared with the first operation mode, a low voltage V_(L) is applied to the first terminal N₁ of the two-terminal photosensing transistor Q₁ and a high voltage V_(H) is applied to the second terminal N₂ for the second operation mode. Because both the control terminal of the two-terminal photosensing transistor Q₁ and the second terminal N₂ are coupled to the high voltage V_(H) (MOS diode), the two-terminal photosensing transistor Q₁ is served as a diode in the second operation mode and produces the diode current I_(diode) (i.e. forward conducting current) through the second terminal N₂ to the first terminal N₁.

FIG. 2B shows the relationship between the diode current I_(diode) and the voltage V_(N2) under the second terminal of the two-terminal photosensing transistor in the second operation mode. Similar with the general diode, the diode current I_(diode) of the two-terminal photosensing transistor Q₁ is zero at the beginning, and then exponentially increases after the two-terminal photosensing transistor Q₁ is turned on. After the two-terminal photosensing transistor Q₁ is conducted (V_(N2)>10V), whether the two-terminal photosensing transistor Q₁ receives or doesn't receive the incident light hv, will not change the existence of the diode current I_(diode), flowing through the second terminal N₂ to the first terminal N₁. It should be noted that the photosensing current I_(photo) with incident light hv (the square point as shown) is bigger than that of the current I′_(photo) without incident light hv (the diamond point as shown). In one embodiment, the diode current I_(diode) is around 1.0E-09 A if the two-terminal photosensing transistor Q₁ receives the incident light and the second terminal V_(N2) is 15V. Conversely, if the two-terminal photosensing transistor Q₁ has not received incident light and the second terminal V_(N2) is at 15V, the diode current I′_(diode) will be around 0.5E-09 A. Therefore, the second operation mode has two functions: the first is to determine whether the two-terminal photosensing transistor Q₁ has received the incident light by detecting/determining the magnitude of the diode current; and the second, is to discharge the second terminal N₂ by the diode current Generally speaking, because the magnitude of the diode current I_(diode) is much bigger than that of the photosensing current I_(photo) (around 1.0E+03˜1.0E+04 units), the process of discharging the second terminal N₂ by the diode current in the second operation mode is comparatively faster than that of charging the first terminal N₁ from the photosensing current in the first operation mode.

FIG. 3 is a schematic view showing the active photosensing pixel in one embodiment. In this embodiment, the active photosensing pixel P₂₂ comprises a two-terminal photosensing transistor Q₁, a driving transistor Q₂ and a reset transistor Q₃. The active photosensing pixel P₂₂ is coupled between the first selection line Sel_2 and the output line Out_2 perpendicular to the first selection line Sel_2.

In the FIG. 3, the two-terminal photosensing transistor Q₁ has a first terminal N₁ coupled to the first node X₁, a second terminal N₂ coupled to the first selection line Sel_2 and a control terminal connected to the second terminal N₂. The driving transistor Q₂ has a first terminal coupled to the first reference voltage V_(ref1), a second terminal coupled to the output line Out_2 and the control terminal connected to the first node X₁. The reset transistor Q₃ has a first terminal connected to the first node X₁, a second terminal coupled to the second reference voltage V_(ref2) and a control terminal coupled to the second selection line Sel_3.

Following, is a description of the photosensing measure. As the sequence diagram of the selection lines and the waveform of the first node X₁ shows in FIG. 4, the waveform of the first node X₁ comprises two situations for the active photosensing pixel P₂₂; one is receiving the incident light hv; and the other is not receiving the incident light hv. In FIG. 4, the solid lines represent the sequence diagram of the first selection line Sel_2 and the second selection line Sel_3, respectively. The break line represents the voltage waveform of the first node V_(X1) when the active photosensing pixel P₂₂ has received the incident light hv, and the dotted line shows the voltage waveform of the first node V′_(X1) when the active photosensing pixel P₂₂ has not received incident light.

The following is a discussion about the operation mode of the two-terminal photosensing transistor. During the first cycle T₁ (known as the exposure and readout cycle), the first selection line Sel_2 is pulled up to a high voltage (such as a high driving voltage V_(GH)) which is higher than of the first node X₁, and the two-terminal photosensing transistor Q₁ is served as a diode and produces the diode current I_(diode) according to the incident light hv and further charges the first node X₁. For example, when the active photosensing pixel P₂₂ receives the incident light hv, the two-terminal photosensing transistor Q₁ will produce a diode current I_(diode) according to the incident light hv, and further charge the first node X₁ to a high voltage V_(X1). When the voltage V_(X1) is higher than that of the threshold level of the driving transistor Q₂, the latter will be conducted by the voltage V_(X1) and produce an output current to the output line. Out_2; hence, the first cycle T₁ is also a readout cycle. On the other hand, during the first cycle T₁, if the level of first selection line Sel_2 is pulled up to be higher than that of the first node X₁ (such as a high driving voltage V_(GH)) and no incident light irradiates on the active photosensing pixel P₂₂, the first node X₁ will be charged to the high voltage V′_(X1) through the diode current I′_(diode) of the two-terminal photosensing transistor Q₁. From the description above, it is clear that the second operation mode of the two-terminal photosensing transistor Q₁ has been employed. Note that when the incident light irradiates on the two-terminal photosensing transistor Q₁, the intensity of the diode current I_(diode) is greater than that of the diode current I′_(diode) which has no incident light irradiating thereon, and then the two-terminal photosensing transistor Q₁ here is served as a diode. Therefore, designing the threshold level of the driving transistor Q₂ between the V_(X1) and V′_(X1), that corresponds to the diode current I_(diode) of the incident light and the diode current I′_(diode) having no incident light, respectively, enables the driving transistor Q₂ to be able to be conducted by the diode current I_(diode) (having incident light) other than by the diode current I′_(diode) (not having incident light), so that whether the incident light hv irradiates on the active photosensing pixel P₂₂ may be determined. Namely, the driving transistor Q₂ can be turned on when the voltage on the first node X₁ is V_(X1), and can not be turned on when the voltage on the first node X₁ is V′_(X1).

During the second cycle T₂ (normally named reset cycle), the level of first selection line Sel_2 is pulled to be lower than that of the level of X₁ (such as the low driving voltage, V_(GL)), such that the control terminal of the reset transistor Q₃ will receive a high driving voltage V_(GH) from the second line Sel_3 and be conducted. In the meantime, the second line Sel_3 resets the first node voltage V_(X1), to cut off the driving transistor Q₂

For the second cycle T₂, since the driving transistor Q₂ is not conducted, the two-terminal photosensing transistor Q₁ does not produce the diode current, even though the incident light hv is irradiated thereupon. Note that the resetting of the voltage V_(X1) at the first node X₁ is completed by means of resetting the reset transistor Q₃ in this embodiment. In this embodiment, the high driving voltage V_(GH) of the first selection line is 10V, and the low driving voltage V_(GL) of the first selection line is 0V. Subsequently, the high voltage V_(X1) and low voltage V′_(X1) of the first node X₁ is between V_(GH) and V′_(GH), wherein the V_(X1) is higher than V′_(X1). The voltage waveform at the first node X₁ is at least V_(th) _(—) _(Q1) higher than that of the low driving voltage V_(GL), wherein, the V_(th) _(—) _(Q1) is the threshold level of the two-terminal photosensing transistor Q₁.

FIG. 5 is a schematic view of the active photosensing pixel. This embodiment is similar with FIG. 3. As such, for simplification, the circuit and sequence of selection lines will not be described in detail here. It should be noted that the active photosensing pixel P₂₂ may further comprise a sensitivity control capacitor C_(sensitivity), wherein the sensitivity control capacitor C_(sensitivity) has a first terminal connecting to the first node X₁ and a second terminal connecting to the third reference voltage V_(ref3).

Following is the description for the function of the sensitivity control capacitor C_(sensitivity). According to the descriptions above, by adjusting the voltage (i.e., V_(X1) corresponding to the first node X₁) of the driving transistor Q₂, it can be determined whether the active photosensing pixel P₂₂ has received the incident light. In this embodiment, when diode current I_(diode) is high (meaning ΔV_(X1) is greatly differed), a sensitivity control capacitor C_(sensitivity) with larger capacitance is employed, and when a diode current I_(diode) is low (meaning ΔV_(X1) is differed slightly), the sensitivity control capacitor C_(sensitivity) with smaller capacitance is employed. Therefore, even if the incident light hv is weak, resulting in a low diode current I_(diode), the driving transistor Q₂ can still sense the weak incident light hv by employing a sensitivity control capacitor C_(sensitivity) with small capacitance. Thus, compared with the conventional method, this embodiment has a better signal-to-noise ratio (SNR).

FIG. 6 is a schematic view showing the active photosensing array and a sequence diagram of the corresponding selection lines. The active photosensing array M_(photosensing) comprises a plurality of selection lines Sel_1-Sel_4, a plurality of output lines Out_0-Out_3, a plurality of active photosensing pixels P₁₁-P₄₃, a driving circuit 50 and a sensing circuit 51. In this embodiment, each active photosensing pixel P₁₁-P₄₃ is similar with those of the embodiment in FIG. 5, in general. For simplification, the description of the circuit connection and the sequence diagram of the selection lines Sel_1-Sel_4 will not be described in detail. The driving circuit 50 may enable the selection signal to the selection lines Sel_1-Sel_4 in sequence. For example, for the first cycle T₁ (exposure and readout cycle), the First selection line Sel_2 is pulled up to be higher than that of the first node X₁ (i.e. high driving voltage V_(GH)). Subsequently, the two-terminal photosensing transistor Q₁ is served as a diode and produces the diode current I_(diode)/I′_(diode) according to the intensity of the incident light hv, such that the first node X₁ is charged to the high voltage V_(X1)/V′_(X1). When the voltage of the first node X₁ is higher than that of the threshold level of the driving transistor Q₂, the latter is conducted and produces an output current to the output line Out_2. Afterwards, the sensing circuit 51, by means of detecting or determining the output current, determines whether the active photosensing pixel P₂₂ has received the incident light hv. Thus, the first cycle T₁ is also the readout cycle. The threshold level of the driving transistor Q2 is designed to be between the voltages V_(X1) and V′_(X1) corresponding to the diode current I_(diode) of the incident light and the diode current I′_(diode) having no incident light, respectively. Hence, the driving transistor Q2 is turned on under the circumstance that the first node X₁ is V_(X1), and is not turned on while the first node X₁ is V′_(X1).

Furthermore, for the second cycle, the voltage level of the selection line Sel_2 is pulled down to be lower than that of the first node X₁ (such as the low driving voltage V_(GL)), and the driving transistor Q₂ resets the latter by resetting the reset transistor Q₃ and the second selection line Sel_3 to turn off the driving transistor Q₂. Note that when the first selection line Sel_2 is pulled high under the first cycle T₁, the selection lines Sel_1, Sel_3 and Sel_4 will be pulled low under the second cycle T2. In other words, the scan array corresponding to the selection lines Sel_1, Sel_3 and Sel_4 will be turned off, so that the neighboring selection lines Sel_1 and Sel_3 of the first selection line Sel_2 will not interrupt the first selection line Sel_2. In other embodiments, the active photosensing array M_(photosensing) includes at least four selection lines, above three scan lines and more than twelve photosensing pixels. Persons skilled in the art may design the active photosensing array M_(photosensing) in accordance with product needs.

FIG. 7 is a schematic view showing a display with an active photosensing array and a sequence diagram corresponding to the selection lines. The display with the active photosensing array M_(sensing-display) comprises a plurality of selection lines Sel_1-Sel_4, a plurality of output lines Out_0-Out_3, a plurality of data lines data_1-data_4, a plurality of active photosensing and displaying units U₁₁-U₄₃, a driving circuit 50, a sensing circuit 51 and a data driving circuit 52. As shown in FIG. 7, each of the active photosensing and displaying units U₁₁-U₄₃ comprises an active photosensing pixel (as P₂₂) and a display pixel (as S₂₂), and thereamong, each of the active photosensing pixels can be installed in accordance with the above-said embodiments of this invention. Therefore, detailed descriptions of the active photosensing pixel and display pixel will be omitted here for brevity. With reference to the display pixel S₂₂, it comprises a switch transistor Q₄ and a liquid crystal capacitor C_(LC), and thereamong, the switch transistor Q₄ comprises a first terminal coupled to the second data line data_2, a second terminal, and a control terminal coupled to the first selection line Sel_2. Meanwhile, the liquid crystal capacitor C_(LC) comprises a first terminal coupled to the second terminal of the switch transistor Q₄ and a second terminal coupled to a third reference voltage V_(ref3).

The following is the discussion for the operation of the display with the active photosensing array. For example, for the first cycle, the voltage level of the selection line Sel_1 is pulled up to be higher than that of the first node X₁ of the photosensing pixel P₂₂ (i.e., the high driving voltage V_(GH)). Subsequently, the two-terminal photosensing transistor Q₁ is served as a diode and produces the diode current I_(diode)/I′_(diode) according to the extent of the incident light hv, so that the first node X₁ is charged to a high voltage V_(X1)/V′_(X1). When the voltage of the first node X₁ is higher than that of the threshold level of the driving transistor Q₂ voltage, the driving transistor Q₂ is turned on and produces an output current to the output line Out_2, and then, the sensing circuit 51, by means of detecting or determining the output current, determines whether the active photosensing pixel P₂₂ receives the incident light hv. Therefore, the first cycle T₁ is also the readout cycle. The threshold level of the driving transistor Q2 voltage is designed to be between the voltages V_(X1) and V′_(X1) corresponding to the diode current I_(diode) of the incident light and the diode current I′_(diode) having no incident light, respectively. Hence, the driving transistor Q2 is turned on when the voltage level of the first node X₁ is V_(X1) and turned off when the voltage level of the first node X₁ is V′_(X1).

Subsequently, during the second cycle T₂, the voltage of the selection line Sel_1 is pulled down to be lower than that of the first node X₁ (such as the low driving voltage V_(GL)). Because the first selection line Sel_2 is enabled by the driving circuit 50, the reset transistor resets the voltage of the first node V_(X1) in order to turn off the driving transistor Q₂. In addition, the driving circuit 50 enables the first selection line Sel_2, so that the switch transistor Q₄ of the display pixel S₂₂ is turned on. Therefore, the display pixel S₂₂ displays images according to the data received from the second data line data_2.

The embodiments provide an active photosensing pixel and photosensing method. Compared with the conventional passive photosensing pixel, the photosensing pixels in the embodiments have higher signal-to-noise ratio and driving ability, which can meet the needs of large display dimensions. Moreover, the control terminal of the two-terminal photosensing transistor Q₁ is connected to the second terminal; therefore, variations in threshold level voltages of the two-terminal photosensing transistor Q₁ will not influence the display device. In the embodiments, the photosensing pixels and array can be disposed at the backplane of the display device and replace the conventional charge coupled device (CCD) photo sensor and the CMOS photo sensor.

While the invention has been described by way of example and in terms of the embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

What is claimed is:
 1. An active photosensing pixel, comprising: a two-terminal photosensing transistor, having a first terminal coupled to a first node, a second terminal coupled to a first selection line and a control terminal connected to the second terminal; a driving transistor, having a first terminal coupled to a first reference voltage, a second terminal coupled to an output line and a control terminal connected to the first node; and a reset transistor, having a first terminal connected to the first node, a second terminal coupled to a second reference voltage and a control terminal coupled to a second selection line, wherein, during an exposure and readout cycle, a level of the first selection line is pulled up to be higher than that of the first node and the level of the second selection line is maintained at a low driving voltage, and the two-terminal photosensing transistor is served as a diode to produce a diode current according to an incident light, wherein the first node is charged by the diode current, such that the driving transistor generates an output current to the output line.
 2. The active photosensing pixel as claimed in claim 1, further comprising a sensitivity control capacitor, having a first terminal connected to the first node and a second terminal connected to a third reference voltage.
 3. The active photosensing pixel as claimed in claim 1, wherein, during a reset cycle after the exposure and readout cycle is completed, the level of the first selection line is pulled down to be lower than that of the first node and a level of the second selection line is pulled up to a high driving voltage, such that the reset transistor resets the first node to turn off the driving transistor.
 4. The active photosensing pixel as claimed in claim 1, wherein the two-terminal photosensing transistor is an N-type a-Si:H TFT.
 5. An active photosensing array, comprising: a plurality of selection lines, including a first selection line and a second selection line; a plurality of output lines; and a plurality of active photosensing pixels, each including: a two-terminal photosensing transistor, having a first terminal coupled to a first node, a second terminal coupled to the first selection line and a control terminal connected to the second terminal; a driving transistor, having a first terminal coupled to a first reference voltage, a second terminal coupled to a corresponding output line and a control terminal connected to the first node; and a reset transistor, having a first terminal coupled to the first node, a second terminal coupled to a second reference voltage, and a control terminal connected to the second selection line, wherein, during an exposure and readout cycle, a level of the first selection line is pulled up to be higher than that of the level at the first node and the level of the second selection line is maintained at a low driving voltage, and the two-terminal photosensing transistor is served as a diode to produce a diode current according to an incident light, wherein the first node is charged by the diode current, such that the driving transistor generates an output current to the corresponding output line.
 6. The active photosensing array as claimed in claim 5, wherein the active photosensing pixels each comprise a sensitivity control capacitor having a first terminal connected to the first node and a second terminal connected to a third reference voltage.
 7. The active photosensing array as claimed in claim 5, wherein during a reset cycle after the exposure and read out cycle, the level of the first selection line is pulled down to be lower than that of the first node and a level of the second selection lines is pulled up to a high driving voltage, such that the reset transistor resets the first node to turn off the driving transistor.
 8. The active photosensing array as claimed in claim 5, wherein the two-terminal photosensing transistor is an N-type a-Si:H TFT.
 9. A photosensing method for an active photosensing pixel, wherein the active photosensing pixel comprises a two-terminal photosensing transistor having a first terminal coupled to a first node, a second terminal coupled to a first selection line and a control terminal connected to the second terminal, a driving transistor having a first terminal coupled to a first reference voltage, a second terminal coupled to an output line and a control terminal connected to the first node, and a reset transistor having a first terminal connected to the first node, a second terminal coupled to a second reference voltage and a control terminal coupled to a second selection line, comprising: providing a first voltage and a second voltage to the first and the second selection lines, respectively, during an exposure and readout cycle, such that the two-terminal photosensing transistor is served as a diode, wherein the first voltage is higher than that of the second voltage; generating a diode current to charge the first node when the two-terminal transistor receives an incident light, such that the driving transistor is turned on according to a voltage level of the first node to produce an output current to the selection lines; and providing the first and the second voltage to the second and the first selection lines, respectively, during a reset cycle after the exposure and readout cycle is completed, such that the reset transistor resets the first node to turn off the driving transistor.
 10. The photosensing method as claimed in claim 9, wherein the first voltage is higher than that of the level of the first node.
 11. The photosensing method as claimed in claim 9, wherein the second voltage is lower than that of the first node.
 12. The photosensing method as claimed in claim 9, wherein the two-terminal photosensing transistor is an N-type a-Si:H TFT. 